The invention relates to a device and methods for the characterisation of a flowing substance. Particular embodiments of the invention relate to the use of the device for the identification of a flowing substance, for controlling the flow of a fuel or combustion gas to deliver a controlled heat of combustion and for measuring the heat capacity of a gas. Further embodiments of the invention relate to a flow control device for controlling the flow rate of a flowing substance and a method for the combustion of a fuel or combustion gas.
Flow control devices are used to deliver a controlled amount of a flowing substance. This works well in most situations, where the nature of a flowing substance is known and substantially constant in composition over time. However, there are also situations where the nature of the flowing substance is not known and can vary in time. Such a situation occurs for example in natural gas distribution networks where different suppliers pump natural gas of different origin and/or composition into the network to various customers. The gas supplied to the customers can vary over time in composition and heat of combustion. The flow of the gas changes in an unknown way because of changes in the viscosity, the density or heat capacity of the various different gasses. In such a situation, most flow control devices, like thermal flow controllers, cannot deliver an accurately controlled mass flow. Because of these unknown changes in the mass flow and heat of combustion, problems can occur like the incomplete combustion of the gas, a too low energy recovery and emission problems.
Another example of such a situation, and a new application of the invention as described hereafter, is the distribution of fuels for engines. For optimum engine performance and fuel economy it is desirable that the flow control is able to deal with varying and/or unknown fuel types and compositions. This will become more and more relevant in view of the increasing desire to further optimise fuel economy, to minimise damaging exhaust gasses and in view of the expected increasing diversification in fuels. Hence, there is a desire for a device for the characterisation of the flowing substance.
Direct and accurate measurement of combustion energy is achieved by actually combusting the gas and measuring the heat. Well known devices for the combustionless characterisation of flowing substances, like chromatography or spectrometry, are accurate but have the disadvantages that they are difficult to integrate in-line, that the response time for characterisation of the substance is still too long for accurate control of the flow and that the devices are too expensive and too big for many applications.
DE 41 18 781 describes a device for the characterisation of a gas, in particular for the determination of the Wobbe number and/or heat of combustion of a gas, comprising a pressure control, a lamellar pressure resistance for isothermal expansion of the gas and a mass and a volume flow rate measurement unit. It is described that the gas cools a heating wire with constant over-temperature and the heating power supplied is used as a measure for the mass flow. Apart from that, the volume flow rate is measured. Further, in the device the gas is expanded isothermally over a flow resistance with laminar flow giving rise to a pressure drop. The pressure drop over the flow resistance; the mass and the volume flow measurement unit is measured, which correlates approximately with the density of the gas. The Wobbe number and/or the heat of combustion are evaluated from the measured mass flow, volume flow and pressure drop by substituting the values in an empirical fit-formula, in which the constants are determined by calibration.
EP-A-0469649 describes a calorimeter in which a laminar flow type flow meter for measuring volume flow is put in series with a thermal type flow meter as described above for measuring a mass flow, in which the gas flow is controlled by controlling a constant temperature difference over the sensor and the caloric value of the fuel gas is evaluated from the pressure difference over the volume flow meter. Because this calorimeter requires two flow meters it is rather complex, expensive and bulky. Further, the accuracy and reliability are insufficient.
FR 2818746 describes a process for the evaluation of a Wobbe parameter wherein a combustion gas is compressed and subsequently expanded and wherein on expansion the flow, pressure and temperature characteristics are measured and wherein a reference gas is similarly measured for-determining reference values for accurately evaluating the Wobbe index from an empirical fit formula.
Similarly, in EP-A-0715169 the pressure of a defined volume of gas is raised to a defined pressure level and after that relaxed to the original state, during which relaxation the density and/or the viscosity values are determined, which are input to an empirical fit formula for evaluating the caloric value and/or Wobbe index.
WO 93/08457 and EP 0554095 describe an improved apparatus and methods to derive from multiple measured characteristics of a fuel gas the desired fuel characteristic like for example a Wobbe index.
The problem of the described devices is that they are all relatively complex, expensive and bulky. In particular, it is a disadvantage that the devices are deigned only for the evaluation of heat of combustion of a fuel gas. More importantly, the devices are often too inaccurate and lack reproducibility for more demanding applications.